Work machines, such as loaders are often configured with the engine behind the back wheels and the loader portion extending beyond the front wheels. This arrangement allows the weight of the engine to offset the load carried in the bucket of the loader portion. Such an arrangement allows for a significant variation in the amount of weight carried by each of the axles. This in turn is expressed by a varying effective diameter of the wheels and the torque needed to move the loader as the torque is individually applied to each wheel.
The power train in a conventional four-wheel drive loader includes a prime mover, and a device to mechanically couple the prime mover with the various wheels. Conventional power train configurations control excessive wheel spin by mechanically constraining the wheels, either laterally or transversely, or both. This is accomplished by forcing the rotation of the wheels by locking a differential, braking the spinning wheel (U.S. Pat. No. 6,631,320) or by slipping a clutch between the slipping wheel and the differential (U.S. Pat. No. 5,265,705). Under some circumstances wheels are allowed to rotate at different speeds, for instance, while the vehicle is turning or if the tires are of different radii. The disadvantage of the differential lock approach to traction control is that it cannot both reduce excess wheel spin and allow the wheels to rotate at different speeds. A disadvantage of using a brake to reduce wheel spin is that this reduces efficiency and causes extra wear on the brakes and requires independently applied brake systems. The disadvantage of using the clutch to reduce the torque to the slipping wheel is that it requires an additional initial and maintenance cost of the clutch and that the clutch wears as it slips.
LeTourneau has a loader with independent electrically driven wheels, and employs a traction control system that compares each wheel speed against the slowest wheel speed, or against the average wheel speed (U.S. Patent Application Publication No. 20070193794). In the automotive industry, traction is controlled using brakes and/or engine torque reduction methods (U.S. Pat. No. 5,025,882). The traction control effort can be a function of the difference between a wheel speed and a reference speed, the rate change of slip (U.S. Pat. No. 5,025,882), and even an estimation of the roads friction characteristics (U.S. Pat. No. 5,504,680). There are methods in the automotive industry for estimating vehicle speed, including the use of an accelerometer (U.S. Pat. No. 4,884,650), by comparison to undriven tires (U.S. Pat. Nos. 5,492,192 and 5,429,428), adjusting for a change in steering (U.S. Pat. No. 5,429,428), or even a model based predictor using wheel speeds and engine information (U.S. Pat. No. 6,560,539). Due to the cost and potential for failure of each sensor, it is advantageous to use fewer sensors to estimate the reference speed. Model based methods use predetermined estimates of unmeasured model parameters, such as tire radius and vehicle weight. Algorithms may be employed to adjust predetermined parameters that are timed, to account for changes in tire wear or payload. Unlike an automobile, a loader's tire diameter and payload change significantly and dynamically during the course of operation making parameter estimation very difficult.
What is needed in the art is a method and apparatus to control the traction of the wheels of a loader with independent drives.